The present disclosure herein relates to a method for measuring a particle beam, and more particularly to a method of measuring a depth profile of a particle beam.
Typically, a proton therapy is advantageous in that unnecessary radiation dose for a normal tissue may be reduced, as opposed to an existing radiation therapy. Nevertheless, the proton therapy is disadvantageous in that it is not easy to figure out a dose, or a depth profile or range of a particle beam. When a dose distribution of a particle beam in a body is not accurately known, a therapy plan system may not accurately calculate a dose of the beam to be exposed. For that reason, in a current proton therapy facility, the therapy is proceeded with a margin of an additional planning target volume (PVT) around a therapy site in consideration of safety of a patient. Since a proton beam passes into a human body deeply as much as energy of its own, completely delivers the energy and then is absorbed, it is not possible to predict an internal dose by an exit dose of a proton beam distribution. Even though a positron emission tomography (PET) imaging method has been proposed in which a position at which a positron generated by a proton interacting with an atom or the nucleus that composes the inner body is pair-annihilated is measured, it is pointed out that it is not suitable to check, in real time, a distribution of positron emitting bodies due to the long half-life of the positron generated by nuclear reaction, and a correlation between a dose distribution of the proton beam and a generation position of the positron emission body is small.
On the other hand, there are some cases where the proton beam collides with the nucleus of an atom. In that case, the proton beam loses energy after the collision with the nucleus and the nucleus emits a deuteron, triton, or a heavy ion, or one or more neutrons in some cases. In this process, the nucleus having received energy from the proton emits a gamma ray of high energy (3 to 10 MeV), while transitioning to an excited state and then decaying to a ground state. The gamma ray in such a case is named as a prompt gamma ray after a phenomenon that an emission occurs as soon as a nuclear reaction takes place. As a correlation between a distribution of the prompt gamma ray and the dose distribution of protons is disclosed, a device using the same is being actively developed and a device in a clinical trial stage is also reported.
On the other hand, the proton continuously loses energy in a process where the proton travels the inner body and performs inelastic Coulombic interactions with electrons around an atom. In this process, a phenomenon that the electrons lose energy and are scattered outside the atom appears. It is very well known that when the electron obtains energy, most of the energy is converted to heat energy, and when a temperature change is induced at a specific position or in a space, a sound wave is generated and spreads to surroundings. Recently, there comes an idea of measuring a Bragg peak position and dose information by measuring a sound wave that is generated as a result of interaction of a proton with an electron. When a proton is injected into the body of a patient, an acoustic signal generated in the body spreads at 360 degree angle and reaches the skin. At this point, when a sound sensor is made to physically contact the skin and a correlation is calculated between a time when the proton reaches the skin and a time when the acoustic signal is measured in consideration of a propagation speed of the acoustic signal in the body, the Bragg peak position may be accurately found. However, it is disadvantageous that the number of protons used in the therapy is limited and the intensity of an acoustic signal generated thereby is not so strong to be measured through the skin.